1. Field of the Invention
The present invention relates, in general, to vibration sensors and, more particularly, to vibration sensors and related control apparatus for detecting machine tool and process failures.
2. Background Description:
Machine tools, such as drill presses, lathes, milling machines, etc., generate characteristic vibrations during normal operation. A change in the characteristic vibrations resulting from wear, part failure, etc., generate vibrations of different frequencies which can be detected and analyzed to provide an early warning of machine or tool failure so that preventive action can be taken to prevent catastrophic damage to the machine tool and/or workpiece.
Vibrations are generated from continuous cutting operations or from impact events, such as pressing, stamping or forming. Plastic deformation, fracture or friction events commonly found in metal working processes generate acoustic emission signals or bursts. For example, when a tool, such as a very small diameter drill bit or tip, begins to fail, microscopic cracks form on the tool body. These signals propagate quickly and generate distinctive acoustical energy signals in the range of 50 KHz to 500 KHz. Vibrations characteristic of individual cutting tooth disturbances that develop as inserts fail on large diameter multi-tooth milling cutters as well as bearing failure in machine tables, or spindles generate vibrations in a different range of approximately 1 Hz to 10 KHz.
Current technology utilizes separate sensors to detect the machine operating vibration and tool breakage vibration. Typically, a low frequency mechanical vibration sensor, such as an accelerometer, is mounted in a housing which is affixed on a machine tool surface, such as the bed of the machine tool, for example. The higher frequency sensors are typically an acoustic emission sensor, such as piezoelectric element, mounted in a separate housing. Further, the different technologies employed with piezoelectric elements and accelerometers have resulted in each type of sensor being manufactured by a different company.
In use, the vibration sensor, whether an accelerometer or an acoustic emission piezoelectric element sensor, is mounted in a housing and affixed to the machine tool. A short length cable, typically two meters or less, connects the sensor output to an amplifier mounted in a separate housing. The short length cable requires that the amplifier housing be mounted within the machine tool environment in relatively close proximity to the machining operation thereby exposing the amplifier housing to metal shavings, coolant, etc.
A cable connects the output of the amplifier to a remote tool monitoring system which is capable of analyzing a certain frequency range of vibrations associated with abnormal machine operation, drill bit breakage, etc. When a selected frequency is detected associated with abnormal operation or tool breakage, an alarm and/or an output signal is generated by the tool monitor which can be used by the machine control to shut down the machine before further damage results to the machine itself or parts are made which do not meet specifications due to the broken tool, drill bit, etc.
The assignee of the present invention has constructed an amplifier circuit with interchangeable, plug-in, bandpass filter circuits, each of selected frequency ranges, such as 1-10 KHz, 0-600Hz, 30 K-500 KHz, and 200 K-400 KHz. The output from each individual sensor is split into two channels, each having a replaceable bandpass filter circuit of a different frequency range. Thus, in the case of the higher frequency acoustic emission sensor, the higher bandpass frequency circuits are employed. The lower frequency circuits are used for monitoring machine operating vibrations.
However, the previous techniques employed to measure both machine operation and machine tool breakage have not been without disadvantages. The mounting of a single vibration sensor in a single housing necessarily requires two separate housings and the associated labor to mount the housings to the machine in order to detect the high and low frequency vibrations associated with cutting element breakage and abnormal machine operation. Each separate sensor housing also requires a separate cable and a separate amplifier housing thereby requiring additional labor and cost.
Further, the amplifier housing is mounted within the machine tool operating environment due to the short length cable employed between the amplifier housing and the sensor. This short length cable has been used even though impedance transducers have been employed in the sensor housing to generate a low impedance output which would enable cables longer than two meters to be employed between the sensor and amplifier. This results in the need for a repairman to enter the machine tool environment after the machine has been turned off and production halted in order to service the amplifier, such as to change the bandpass frequency circuit in the event of an operation change.
Thus, it would be desirable to provide a vibration sensor apparatus which overcomes the deficiencies found in previously devised vibration sensor devices. It would be desirable to provide a vibration sensor apparatus which has a single sensor housing containing multiple vibration sensors for detecting different vibration frequencies. It would also be desirable to provide a vibration sensor which provides a large contact surface between the machine and the vibration sensors for increased vibration detection. It would also be desirable to provide a vibration sensor apparatus which utilizes longer length cables between the single sensor housing and a single amplifier housing so as to enable the amplifier housing to be mounted remote from the machine tool operating environment for easy access by service personnel without significantly interrupting machine operation.